1. Field of the Invention
The present invention relates to a sodium secondary battery which is sealed in an improved manner, and in particular to a rechargeable secondary battery applied to batteries for storage of electric power for road grading and to electric vehicles.
2. Background Art
FIG. 9 (PRIOR ART) is a schematic illustration of a conventional sodium secondary battery.
As shown in FIG. 9, in the conventional sodium secondary battery, a negative electrode chamber is formed by placing sodium 3 into a bottom-closed, hollow cylindrical solid electrolyte 2 provided inside an outer case 1; and between the outer case 1 and the solid electrolyte 2 is disposed a porous electrode 4 impregnated with sulfur 5 serving as a positive electrode active substance, to thereby form a positive electrode chamber. An outer case metal fitting 6 having an L-shaped cross section is welded to the opening portion of the outer case 1. For a cover 12, a metal fitting (hereinafter referred to as a cover metal fitting 8) is provided. An electric insulator 7, attached along the outer periphery of the solid electrolyte 2 in the vicinity of its opening, is sandwiched between the outer case metal fitting 6 and the cover metal fitting 8 via an aluminum alloy 9 serving as a brazing material, and undergoes hot-pressing to thereby provide sealing of the structure.
With the above structure, in the discharge process the sodium 3 contained in the negative electrode chamber dissociates into sodium ions and electrons. The sodium ions pass through the solid electrolyte 2 to migrate into the positive electrode chamber outside the solid electrolyte and are combined with the sulfur 5 and electrons circulating outside the cell to thereby form sodium polysulfide.
Meanwhile, in the charge process, sodium polysulfide present in the positive electrode chamber dissociates into sodium ions, electrons, and sulfur. The formed sodium ions pass through the solid electrolyte 2 to migrate into the negative electrode chamber defined by the inside of the tubular solid electrolyte and are combined with electrons circulating outside the cell to thereby form sodium 3.
The process for manufacturing the above cell will next be described.
(1) The insulator 7 is bonded to the upper portion of the bottom-closed, tubular solid electrolyte 2 by use of a glass solder 10. PA0 (2) The upper face of the insulator 7 is bonded to the cover metal fitting 8, and the lower part of the insulator 7 is bonded to the outer case metal fitting 6, both by hot-press bonding by the mediation of aluminum alloy 9 serving as a brazing material. As used herein, the term "hot-press bonding" refers to bonding between heterogeneous materials by the application of pressure in an atmosphere of about 600.degree. C., which is close to the melting point of aluminum alloy 9. PA0 (3) The positive electrode 4 impregnated with the sulfur 5 serving as a positive electrode active substance is placed in the outer case 1, and then the outer case metal fitting 6 is welded to the outer case 1. PA0 (4) A wick 11, which also serves as a safety tube and has a sodium discharge outlet 11a, is secured onto the cover 12, which is then welded with the cover metal fitting 8. PA0 (5) The sodium 3, in the form of liquid, is injected from a sodium-injection-hole, and the hole is sealed with a sealing member 13. PA0 (1) A high temperature is required for melting the aluminum alloy 9 serving as a brazing material; thus, a heating apparatus, such as an electric furnace achieving a temperature as high as approximately 600.degree. C., is required. A vacuum condition may also be required, depending on the bonding method employed. PA0 (2) Under the aforementioned conditions (i.e., high temperature and in vacuo), pressurization must be performed, which raises disadvantages associated with scaling up of the apparatus employed and an increase in the number of manufacturing steps, such as cooling from high temperatures and raising pressure from the vacuum condition to atmospheric pressure. PA0 (3) There may be a case in which .beta.-alumina, serving as an insulator, breaks due to high temperature. The breakage induces reaction between sodium and sulfur to suddenly cause a high temperature condition. When the temperature is higher than the melting point of aluminum, the cell is broken. PA0 (4) Conventional planar-type sodium secondary batteries suffer a problem of poor sealing caused by a large proportion of hot-press-welded parts since flanges located at the periphery of a positive electrode container and a negative electrode container being opposite to each other are hot-press welded. PA0 i) A secondary battery can be manufactured without thermal stress being generated and with remarkably increased yield; PA0 ii) An electric furnace is eliminated from the manufacturing facility, whereby the time for heating in the electric furnace is saved, a cooling step may be omitted, manufacture is simplified, and the facility cost is reduced remarkably. PA0 iii) Since the solid electrolyte 2 will not be broken, possibility of disintegration of the brazed members is reduced even when the temperature of the brazed portion becomes higher than the melting point of aluminum as a result of reaction between sodium and sulfur; and PA0 iv) The battery is easily disassembled by simple loosening of the bolts, to thereby facilitate recycling the battery, which is more difficult in the case of batteries fabricated through melt bonding.
Problems that arise in relation to hot-press bonding for manufacturing the sodium secondary battery will next be described.